The controllable parameters in a continuously variable transmission differ from a stepped automatic transmission. The controllable parameters in the stepped automatic transmission consist of two independent parameters: the moment of the gearshift and the kind of gearshift (upshift or downshift).
Compared to that, in a continuously variable gear three independent parameters have to be taken into account: a starting point (for example, an output speed or output transmission ratio), an objective (for example, an end speed or a desired transmission ratio) and the temporary variation between starting point and objective.
In the requirement profile of a driving strategy for a continuously variable gear, the crucial points for good driveability are a favorable consumption and an application expenditure utilizable in the practice. In the conversion of a driving strategy, the resource requirement must be small. The driving strategy must automatically adapt to different driving situations (driver, vehicle and environment). In case of errors common in the practice, the system must react insensitively, that is, possess a certain sturdiness.
Within the crucial points, in turn, a series of individual problems are of special significance. In the driveability, it is important to meet requirements such as: a good relationship between engine speed and road speed; engine dynamic and driving performance corresponding to the desired performance; adaptation to the driver's desires; possibility of manual contact for the driver; adaptation to the driving state (for example, city uphill); support of the vehicle brake; integration of the converter into the driving strategy; reliable reverse shifting after low when stopping; and adaptation to the driving usages specific to the country.
Regarding the resource requirements in the conversion little EPROM; little RAM; and a short running period requirement are wanted.
By sturdiness of the driving strategy, to be understood are properties such as: insensitivity to errors of the input signals; and insensitivity when scatterings of the vehicle parameters occur within a vehicle series of a manufacturer.
Important in the section of the requirement profile dealing with consumption are the special aim requirements like: a long hold-up duration in consumption-favorable ranges of the engine performance graph; the target performance of a high engine output at the lowest possible speeds; a reduction of the engine speed during constant drive; and an operating point guidance during coasting corresponding to the coasting disengagement of the engine.
In the application what is important is: reliable, constant setting of an operating point achievable for the driver in all driving situations; optional combination of input parameters; brief application periods at high operational density; and great flexibility when taking into consideration the desires of the vehicle manufacturer.
In the automatic adaptation, the following is concerned: adaptation to the momentary mode of driving; adaptation to the momentary environmental conditions (uphill, downhill, etc.); and adaptation to the momentary driving state (cold or warm vehicle, additional consumption, trailers, etc.).
As a consequence of the requirement profile for a driving strategy in a continuously variable gear, three systematic problems can be formulated by abstraction: the driving strategy must be adaptive; efficient tools must be available for the development; and the application of the driving strategy must be capable of being electronically supported.
Comparing the requirement profile for a driving strategy of a continuously variable gear with the solutions already known, in spite of the multiplicity of known driving strategies, individual problems are solved, but no driving strategy is known that totally satisfies the application profile.
Thus, for example, the setting of an operating point has been satisfactorily solved by taking into account individual input parameters. But this conclusion does not apply to an optional combination of the input parameters.
In the complexity of the requirement profile, the description of the relationships turns out to be correspondingly expensive and thus, also for these reasons, an efficient programming and application is still not possible.
To set an operating point, taken into account are the individual input parameters of the known, electronically controlled continuously adjustable gears, of which (independently of the driver's wish or automatically) it is possible to drive on different characteristic lines of the engine performance graph. The operation takes place either on the characteristic line of an optimal consumption or on a characteristic line of optimal performance. From the characteristic lines of the engine performance graph, shifting characteristic lines in the shifting performance graphs are derived which allow adjustment of the gear to a so-called economy or a sport mode. In the solution given in DE-A 43 12 415, said characteristic lines are interlinked to form a shift performance graph where a boundary line corresponds to a characteristic line of an optimized consumption and another boundary line to a characteristic line of optimized performance. The operating points, between said boundary lines, lie on a so-called dynamic characteristic lines, the position of which depends on the amount of the change of speed of the performance requirement.
The criteria for selecting the shifting performance graph and current dynamic characteristic line, along which the operating point is guided, is thus the rate of alteration of the throttle valve of the engine.
In EP-A 0 657 668, adaptive strategies for optimal adaptation of the shifting performance graphs are divided in a long-time strategy (detection of the type of driver), an intermediate strategy (driving environment) and a short-time strategy (current driving situation).
Depending on the current vehicle type (maximum performance, as well as dependence of the driving resistance on the road speed at medium load), as well as consumption and a sport criteria, two speed-dependent boundary curves of performance are plotted as characteristic lines in the P-v-diagram. A minimum and a maximum rating number is coordinated with said characteristic lines. The current engine performance is determined and compared with the two boundary curves. By the comparison, a current rating number is received which, during travel, is permanently detected. The current rating numbers are subjected to a periodical filtering; hereby a filtered rating number is obtained. With the aid of the filtered rating number, two-dimensional characteristic lines or three-dimensional characteristic line graphs are detected to control the prime mover. The objective of this method is to obtain, for the gear control, only one modified long-time strategy and, when the slip detection is removed from the intermediate-term strategy, to use a short-term strategy.
This known method is advantageous for part of the aspects of a driving strategy, since the current rating number can be found by simple interpolation. The control of the time characteristics by an adequate filter should require no great expenditure. On the other hand, in the known method, it remains open how other no less important aspects of the requirement profile could be taken into account.
To reduce the development periods, it is proposed in the device for control of a continuously variable gear, according to DE-A 44 45 325, to evaluate the engine torque in order to calculate an input torque of a gear. By comparing the calculated input torques of the gear with a nominal driving shaft torque which is based on the parameters that indicate the engine load, a nominal reduction ratio is determined. From the performance graphs of nominal driving shaft torque, a nominal reduction ratio is understood that is compared with the current reduction ratio of the continuously variable gear. From the difference, a correcting variable is derived in order to equate the actual reduction ratio with the nominal reduction ratio. This solution also requires several performance graphs in order to take into account a current mode of driving.
In the device according to DE-A 42 23 967, an output torque of the gear is adjusted within the scope of a preset operating strategy. The presettable operating strategy represents a sport or a consumption-favorable driving characteristic. A performance graph, especially with supporting points, is detected by consumption measurements. The nominal value of the gear transmission ratio is selected as low as possible depending on a minimum speed of the engine and on a desired output torque. The gear transmission ratio remains substantially, at the 50% actuation of the accelerator pedal, at a minimal value and then only rises. With this rough adjustment, an individual application can be omitted under certain circumstances.
International patent applications, WO 90/04119 and WO 94/24463 have disclosed other controls for continuously variable gears which make a limitation of the driving ranges possible in order to improve the driveability.